Spirally wound paperboard tubes are widely used in textile and other industries as cores for winding of filaments, yarns, and other materials such as films as they are produced. Although paperboard is relatively weak on a single layer basis, a tube constructed from multiple spirally wound paperboard layers can attain substantial strength.
In the textile industry, yarn winding speeds have increased dramatically in recent years. Currently available textile winders are capable of operating at winding speeds of up to 8,000 m/min. High winding speeds result in the application of significant forces to the textile cores as is well known in the art. For example, U.S. Pat. No. 3,980,249 to Cunningham et al., issued in 1976, reported the phenomena of disintegrating and exploding high speed textile cores with winding speeds of 12,000 feet per minute (3660 m/min). The significant increases in textile winding speeds since that time have worsened the known problems.
Winders can be drum driven or spindle driven. Drum driven winders employ a driven winding drum having a drive land which circumferentially contacts the surface of the textile core during start up and rapidly increases the surface speed of the textile core to the desired winding speed. Currently available drum winders are capable of accelerating the speed of the textile core from rest to 6,000 m/min in as little as five seconds. Spindle driven winders accelerate the textile core from rest to the desired winding speed at a much lower rate of acceleration using a driven spindle supported coaxially within the interior of the textile core. These winders include a bail roll having a drive land which contacts the surface of the rotating tube under pressure.
The forces exerted on the textile cores particularly during start up of a high speed winding operation thus include compressive forces (head pressure) such as are exerted by contact between the drive land and the face of the textile core; shear and abrasive forces such as are exerted by the driven winding drum during initial acceleration of the textile core surface; tensile forces resulting from circumferential acceleration from rest to start-up speed; radially oriented stresses resulting from the centrifugal force generated by the high rotational speed of the textile core; and circumferential stresses caused by tube rotation.
Although a few carefully designed and constructed paperboard textile cores have been found capable of operating with the 6,000 meter per minute winders, at the present time no commercially available paperboard textile core is capable of consistently rotating for an excess of two minutes on the 8,000 m/min. winder without exploding. This is true of tubes constructed with the best paperboards in the world.
The mechanisms responsible for disintegration of textile tubes during high speed winder start-up are poorly understood, due in part to the nature of the paperboard tubes, themselves. Paperboard tubes are formed of layers which have been adhered together during the manufacturing process. And the paperboard forming each of these layers is an orthotopic material having properties in the lengthwise or machine direction (MD) that are different from the properties of the same paperboard in the widthwise or cross-machine direction (CD) due to the tendency for more paper fibers to be aligned along the MD as compared to CD. In addition, paperboard strength properties in the direction perpendicular to the plane of the paper are less than those of the paperboard in either the MD or CD, also due to fiber alignment.
Because the paperboard plies forming the textile cores are spirally oriented, there is no alignment of the paperboard plies in either of the CD or the MD directions, along the axis of the tube or along its circumference. Moreover, even though the theoretically predominant stress generated during high speed tube rotation would appear to be the extremely high circumferential stresses at the interior face of the tube, paperboard is known to have sufficient strength to withstand these forces. And observations of exploding tubes reveal failure near the middle of the tube wall.
Recently, a closed-form elasticity solution has been developed to predict stresses and strains in spiral paper tubes loaded axisymmetrically. In experiments to verify this theory, a load was applied via fluid to the exterior periphery of a spirally wound paperboard tube so that the radial load was uniform around the circumference of the tube; see T. D. Gerhardt, "External Pressure Loading of Spiral Paper Tubes: Theory and Experiment", Journal of Engineering Materials and Technology, Vol. 112 , pp. 144-150, 1990. The theory considered in this work successfully incorporated considerations concerning the orthotopic properties of paperboard tubes. However the dynamic nature of the forces underlying textile core disintegration during high speed winder start-up, and the readily apparent difficulties in replicating these forces under static conditions presents a much more complex set of considerations than those successfully analyzed in the 1990 article.
The angular orientation of spirally wound plies with respect to the tube axis in commercially available textile cores is limited to a relatively narrow range of angles. This is believed to result from manufacturing considerations, the widespread availability of certain standard paperboard ply widths, and the widespread use of textile cores of relatively small standard inside diameters (ID). Currently available textile cores employ spiral winding angle constructions in which the standard ply widths are matched with the desired standard IDs so that known manufacturing efficiencies are increased while manufacturing difficulties are avoided.
Spirally wound tubes are manufactured employing a stationary mandrel. The plies are fed in overlapping relation onto the mandrel and the tube formed on the mandrel is rotated by a belt which moves the tube axially along the mandrel. The angle at which the plies are fed to the mandrel is determined by the outside diameter (OD) of the mandrel and the width of the plies as a result of geometric limitations. Narrower width plies must be fed at a larger winding angle relative to the mandrel (closer to a transverse orientation) while wider plies must be fed at a lower angle (more axially aligned with the mandrel).
The use of wider paperboard plies thus increases the rate of tube formation as a result of different and cumulative effects. Wider plies cover a greater axial length of the mandrel surface simply because they are wider. In addition the lower winding angle that must be used with wider plies provides a closer alignment of the ply with the axis of the mandrel, resulting in a greater axial coverage of the mandrel surface relative to the actual width of the ply. Thus for a given belt speed, the use of wider plies and their corresponding lower wind angles provides a higher tube production rate, i.e., a greater axial length of tube production per minute.
The use of wider plies and their corresponding lower winding angles also simplifies the tube forming process because the plies are fed onto the mandrel in greater alignment with the axial movement of the tube being formed. This in turn, results in a lower friction between the interior surface of the rotating tube and the stationary mandrel. The lower friction between the tube ID and the mandrel can allow for the use of higher belt speeds and can minimize the potential for disruption of adhesion between plies as the tube is rotated around, and moved axially along the mandrel.
With the exception of paperboard tubes of very large IDs, e.g., greater than about one foot, high wind angles are avoided during tube manufacture by the use of wider paperboard plies for the reasons discussed above. With the very large tubes, the large mandrel size dictates the use of high winding angles or the use of extremely wide plies which are not readily available, and which are not readily used with commonly available tube manufacturing equipment. However, standard ID requirements for textile winding cores range from 3 in. (75 mm) up to 5.6 in. (143 mm). Tubes of these IDs can be, and are, manufactured without requiring use of high winding angles and narrow ply widths. Thus, all commercially available textile cores for high speed winders are made using continuous plies having widths of 4 inches or greater and winding angles of less than 74 degrees. Textile cores having diameters in the lower portion of the standard range have winding angles of less than 70 degrees. Textile cores having diameters in the upper part of the standard range use ply widths of at least 5 inches.